1. Field of the Invention
The present invention relates to a structure of a moving member in a vibration wave motor for friction driving the moving member by a travelling vibration wave.
2. Related Background Art
Vibration wave motors which can be driven by a travelling vibration wave have recently been brought into practical use. A known example of such a vibration wave motor will be described with reference to the schematic view of FIG. 4. Referring to FIG. 4, reference numeral 1 denotes an electro-mechanical energy transducer element such as an electrostrictive element, a piezoelectric element or a magnetostrictive element and being made of, e.g., PZT (lead zirconate titanate). A vibration member 2 is made of an elastic material, to which the electrostrictive element 1 is bonded. The vibration member 2 is held together with the electrostrictive element 1 on the side of a stator (not shown). A moving member 3 constitutes a rotor urged against the vibration member 2 to be in contact therewith in this example. A plurality of electrostrictive elements 1 are bonded to the vibration member 2 such that one group thereof is offset from another group by 1/4 the wavelength .lambda. of the vibration wave. The electrostrictive elements in each group are arranged at a pitch 1/2 the wavelength .lambda. such that the polarities of two adjacent elements are opposite.
In the vibration wave motor of this arrangement, assume that an AC voltage of Vo.sin.omega.t is applied to the electrostrictive elements in one group and an AC voltage of Vo.cos.omega.t is applied to the electrostrictive elements in the other group. Then, the polarities of two adjacent elements become opposite, and AC voltages having phases offset by 90.degree. from each other are applied to the two groups to vibrate the elements. The vibration is transmitted to the vibration member 2 to cause a bending vibration in accordance with the pitch of the arrangement of the electrostrictive elements 1. When the vibration member 2 partially projects by bending to correspond to the position of every other electrostrictive element, the positions of the remaining electrostrictive elements are recessed. Meanwhile, the first group of electrostrictive elements is offset from the other group by 1/4 the wavelength .lambda. as described above, allowing travel of the bending vibration. The vibration is energized sequentially while the AC voltage is applied, thereby causing a travelling bending vibration wave to propagate within the vibration member 2.
The travel of the wave in this state is shown in FIGS. 5A, 5B, 5C and 5D. Assume that the travelling bending vibration wave travels in the direction of arrow X1. A central (reference) plane in the vibration member in a stable state is designated as 0. The central plane 0 is in the state indicated by the alternate long and short dashed line in a vibration state, and stress caused by bending competes in a neutral plane 6 of the vibration element 2. Stress does not act on an intersection line 5.sub.1 of the neutral plane 6 and a sectional plane 7.sub.1 perpendicular thereto; instead, only vertical vibration occurs. Simultaneously, the sectional plane 7.sub.1 undergoes pendulum vibration to the right and left about the intersection line 5.sub.1, and plane 7.sub.2 or 7.sub.3 undergoes pendulum vibration to the right and left about an intersection line 5.sub.2 or 5.sub.3 in the same manner.
In the state of FIG. 5A, a point P1 on an intersection line of the sectional plane 7.sub.1 and the surface of the vibration member 2 near the moving member 3 is the right dead point of the right and left vibration and moves only vertically. A force acting on the point P upon the pendulum vibration will be considered. When the intersection line 5.sub.1 is at the positive side (above the central plane 0) of the wave, stress acts to the left (in a direction opposite the travelling direction X1). When the intersection line 5.sub.1 is at the negative side (under the central plane 0) of the wave, stress acts to the right. In other words, when the intersection line 5.sub.2 and the sectional plane 7.sub.2 are positioned in the manner as described above, stress acts on a point P2 in a direction indicated by the arrow in FIG. 5A. When the intersection line 5.sub.3 and the sectional plane 7.sub.3 are positioned in the manner as described above, stress acts on a point P3 in a direction indicated by the arrow. When the wave travels and the intersection line 5.sub.1 is at the positive side of the wave as shown in FIG. 5B, the point P1 moves to the left and upward simultaneously. In FIG. 5C, the point P1 moves to the left at the upper dead point. When the wave further travels, the point P1 moves to the left and downward simultaneous (FIG. 5(d)), and subsequently, to the right and upward simultaneously, thus returning to the state of FIG. 5A.
When the series of above movement are synthesized, the point P1 performs a spheroidal motion. The radius of the spheroid is a function of t where t is the distance between the neutral plane 6 of the vibration member 2 and the surface thereof near the moving member 3.
Meanwhile, the moving member 3 is urged to be in tight contact with the vibration member 2 and the spheroidal motion at the point P1 on the vibration member 2 friction drives the moving member 3 in an X2 direction. All points including points P2 and P3 on the surface of the vibration member 2 on the positive side of the wave friction drive the moving member 3 in the same manner as the point P1. A moving member and a stator vibration member of a conventional vibration wave motor driven in this manner have a considerably large contact area. More specifically, ultra-fine surface finishing is performed so that the surfaces of the stator and the moving member contact each other at a uniform pressure. Alternatively, the contact surface of the moving member may be divided in a circumferential direction or the moving member comprises an elastic rod or plate, so as to sufficiently follow the flat surface of the stator as described in Japanese Patent Application Laid-open No. 188381/1984.
However, when the moving and stator vibration members are brought into contact with a large contact area, problems arise as follows: First, since the distance between the two flat surfaces, i.e., between the moving and stator vibration members changes in a vibration manner, a positive pressure occurs in the air therebetween, thereby floating the moving member with respect to the stator. The larger the opposing areas, or the narrower the gap between the two flat surfaces, the larger the floating force, thereby lowering the frictional force that drives the moving member of the vibration wave motor and resulting in a considerably lower output of the motor. The influence of a floating force in an air film caused by vibration of a surface is described in Kyosuke Ono, "Lubrication"Vol. 21, No. 9 (1976) pp. 589 to 597.
The second problem concerns vibration propagation on the contact surfaces of the stator vibration and moving members. The stator vibration and moving members are solids and have similar specific acoustic impedances compared to that of the ambient air. Therefore, vibration of the stator vibration member can easily propagate to the contact surface with the moving member.
Specifically, when the vibration motor is stopped, the contact surfaces are large since the stator is not vibrating. In this state, the stator vibration member is not an independent vibration system but is coupled firmly with the moving member when it starts the vibration wave motor. Thus, even if the stator alone is to be vibrated, it cannot be vibrated at a resonant frequency of the stator alone, resulting in failure to energize the vibration wave motor.